Federal Highway Administration
1200 New Jersey Avenue, SE.
March 11, 2008
In Reply Refer To: HSSD/CC-69E
Dean L. Sicking, P.E., Ph.D.
Safety by Design Company
5931 The Knolls
Lincoln, NE 68512
Dear Dr. Sicking:
Thank you for your December 14, 2007, letter requesting Federal Highway Administration (FHWA) acceptance of an extension to the Road Systems, Inc., Single Sided Crash Cushion (SSCC) as a test level 3 (TL-3) device for use on the National Highway System (NHS). You requested that we find the extended SSCC acceptable for use on the NHS under the provisions of National Cooperative Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.”
The FHWA guidance on crash testing of roadside safety hardware is contained in a memorandum dated July 25, 1997, titled “INFORMATION: Identifying Acceptable Highway Safety Features.”
The primary purpose of the request is to provide an end treatment option for bridge rail ends placed close to an intersecting street or driveway that restricts the ability to utilize conventional guardrail designs, such as a short-radius guardrail system. There is no NCHRP Report 350 approved short-radius guardrail system and even the older designs extend as much as 50 ft beyond the end of the bridge railing. There are many situations where there is less than 45 ft available for a barrier end treatment. On the other hand, it is desirable to extend the approach guardrail as far as possible to reduce the risk of a vehicle traveling behind the bridge rail while maintaining adequate end-on safety performance. Most such applications could be treated if the SSCC could be extended up to an additional 16 ft.
The SSCC was originally accepted by FHWA in 2002 (CC-69B) in a soil mounted configuration and in 2004 (CC-69D) in a pavement mounted option. Both of these designs incorporate a 27 ft – 11.5 inch long design with a total of 8 breakaway posts. The spacing between the first and second post was 6 feet 6 inches. The next three posts were spaced 4 feet apart, and the final three posts were reduced to 2 feet apart. It is proposed that the SSCC be lengthened by incorporating additional box beam railing in the 4 foot post spacing region. As shown on the enclosed drawing, extensions of 4, 8, 12, and 16 feet are proposed.
No additional testing was conducted on the extended SSCC. You claim that the modification would have no adverse effect on the performance of the SSCC. You enumerated the various crash tests that are required for a barrier terminal:
Test 30, Test 31, Test 32, Test 33 all involve end-on impacts. As the proposed change extends the second stage energy absorber, the only effect would be to increase the terminal’s energy dissipation capacity for end-on impacts.
Test 34 and Test 35 involve impacts on the side. As the proposed extension does not introduce components that would change the terminal’s redirective capacity, there is no need to run either test.
Test 39 is a reverse direction test. As was the case for the same-direction side impacts discussed above, no new design elements are introduced to alter the redirection capability of the device.
We concur in your claim that the optional extension of the SSCC would not adversely affect the crashworthy performance of the device. Therefore, the requested modification is acceptable for use on the NHS, when permitted by a highway or transportation authority, under the range of conditions the original design was tested.
Please note the following standard provisions that apply to the FHWA letters of acceptance:
David A. Nicol
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